tools/kvm_stat: add command line switch '-z' to skip zero records

When running in logging mode, skip records with all zeros (=empty records)
to preserve space when logging to files.

Signed-off-by: Stefan Raspl <raspl@de.ibm.com>
Message-Id: <20200402085705.61155-2-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>